It is known to use gamma radiation for scanning structures, for example to obtain information about the density within the structure or to identify flaws such as cracks or corrosion in the structure. This is particularly useful for inspecting pipes subsea, where it is not always possible to inspect the pipe from the interior. Gamma scanning is also used for obtaining information about other industrial structures such as distillation columns and the like.
An apparatus for scanning structures such as a pipeline or process vessel using gamma radiation is described in GB 2496736 A. This apparatus comprises a source of gamma radiation and an array of detectors spaced apart circumferentially. The apparatus is capable of being arranged with the structure to be scanned, such as a pipeline, positioned between the source and detectors so that radiation emitted by the source can pass along the plurality of paths through a portion of the structure to the detectors. The number of detectors in the array may range from fewer than 10 up to more than 100, e.g. up to 150, depending on the application. Counting the number of gamma photons transmitted from the source to the detectors, through the structure being scanned, enables differences in the density of different parts of the structure to be detected.
To obtain high resolution data, a large number of detectors are used, closely spaced from one another. The detectors are arranged in an arc centred on the structure to be scanned. In operation, the source and array of detectors are arranged in fixed relationship with respect to each other, and are rotated around the structure to be scanned. In this way, information about the density of the structure along a plurality of paths is obtained, enabling a high resolution density tomogram of the structure to be calculated. The apparatus may also be translated axially to scan different sections of the structure. The device can produce accurate, high resolution data, but the data may be slow to acquire. A typical scan at a single axial location, covering perhaps a few mm of axial length of the structure, may take several minutes to complete. There remains a need for a technique that can scan pipes more quickly.
In subsea applications, additional constraints arise. When operating at a depth of 1000 metres underwater, the pressure is 100 atmospheres and increases by a further 100 atmospheres for each additional 1000 metres of depth. The apparatus must be able to withstand this pressure yet remain sufficiently compact for deployment using remotely operated vehicles capable of operating at the required depth.
A typical detector for detecting gamma radiation comprises a scintillating crystal. Gamma rays entering the scintillation crystal interact with the scintillating material to produce photons in the visible and/or ultraviolet region. These scintillation photons are detected using a photodetector, for example a photomultiplier tube, which outputs an electrical pulse providing information about the number and energy of the incident gamma photons.
Some prior art scanning techniques, particularly in medical applications, use pixelated x-ray detectors. Such detectors may feature a scintillating layer converting x-ray radiation into light and a photodiode array transforming light intensity into electrical current. The radiographic image may then by compiled by integrating current over a certain period of time for each pixel in the array and then digitizing the results via analogue to digital converters. Such detectors may have limitations in that they are typically designed for use with electrical x-ray generators that emit a broad energy spectrum peaking at 80-160 keV in many cases. Use of such detectors with high-energy gamma radiation sources is often discouraged as the scintillating layer in the panel does not stop a significant portion of the incoming gamma flux. Such panels also use photodiodes to convert light into electrical current. That may provide good linearity, but can require a very large incoming photon flux, such as may be obtained with continuous x-ray illumination, in order to generate an electrical current above the thermal threshold (so called “dark current”) at room temperature. However, they may not be so suitable for use with low count rates of gamma radiation. There is therefore a need for improved detectors for use in gamma radiation scanning.
Sources of gamma radiation must be transported safely. To achieve that, source containers for highly active sources must be certified, for example Type B (Type B(U) or Type B(M)) certified in accordance with The IAEA Regulations for the Safe Transport of Radioactive Material 2012 Edition Specific Safety Requirements No. SSR-6. Type B certified containers exist in the prior art, but may not be suitable for use at subsea locations.
Preferred embodiments of the present invention seek to overcome one or more of the above disadvantages of the prior art.